The present invention relates to the field of demagnetizing circuits, and in particular to a demagnetizing circuit for demagnetizing color picture tubes.
Color picture tubes must be demagnetized in order to obtain sufficient color purity. For this reason, a demagnetizing coil is used through which a fading high-amplitude alternating current is sent when the equipment is turned on. However, the leakage current flowing through the demagnetizing coil during continuous operation should be as low as possible in order to reduce power dissipation.
In conventional demagnetizing circuits, a positive temperature coefficient (PTC) thermistor in series with the demagnetizing coil is employed for obtaining the decreasing amplitude in the alternating current. The PTC thermistor is a resistor with a resistance that is a function of temperature, wherein the resistance increases as the temperature increases. The resistance of the PTC thermistor is thus very low when the equipment is turned on, that is, when it is cold, but it is substantially higher when it is warmed up in the operating mode.
A problem with a PTC thermistor is that it suffers from the disadvantage that during continuous operation of the equipment, leakage current flowing through the demagnetizing coil and the PTC thermistor causes continuous power dissipation of approximately 2 W. This is particularly troubling in standby mode operation because the power consumption should be as low as possible in that mode of operation. Therefore, in expensive television sets the demagnetizing current, (i.e., the current flowing through the demagnetizing coil and the PTC thermistor) is turned off during continuous operation using an additional circuit (e.g., that includes a triac or optical coupling device).
Therefore, there is a need for a demagnetizing circuit in which the desired current flow can be achieved with a reduced complexity control circuit and without substantial power dissipation occurring during continuous operation.
Briefly, according to an aspect of the present invention, a demagnetizing circuit for controlling a demagnetizing current applied to a demagnetizing coil includes at least two transistors that are controlled by at least one capacitive voltage divider. A rectified alternating voltage is applied to the capacitive voltage divider, which applies control signals to the transistors to control the demagnetizing current supplied to the demagnetizing coil.
The demagnetizing circuit uses MOS or bipolar transistors rather than a PTC thermistor. Thus, with modest complexity in terms of the control, not only can a demagnetizing current with fading amplitude be produced, but the demagnetizing current returns to zero. As a result, after the demagnetizing no power dissipation occurs, which is particularly advantageous when the equipment is in standby mode.
The transistors are controlled via a capacitive circuit that may include a single capacitive voltage divider, or at least two separate capacitive voltage dividers. Ideally, the inverse diode generally present in MOS transistors is also used. When using bipolar transistors that are not equipped with such inverse diodes, discrete diodes must be provided.
In accordance with one exemplary embodiment of the present invention, the complexity of the control can be further reduced when the source and gate terminals for the two MOS transistors are connected to one another so that the demagnetizing circuit can be operated with just one capacitive voltage divider.
In another aspect of the present invention, a demagnetizing circuit may retroactively actuate or activate the demagnetizing even after the equipment has been turned on, so that demagnetizing can also be performed during continuous operation of the equipment. This is particularly desirable when the equipment remains powered up for an extended period and is merely switched to standby outside of operating times. In this embodiment, an additional transistor is used (e.g., a small-signal transistor) and a corresponding voltage must be applied to this additional transistor to switch this transistor to the conducting state to initiate the demagnetizing. For instance, this can occur with a voltage that is low in the equipment standby mode and is high in the operating mode.
The invention is particularly suitable for demagnetizing color picture tubes in television equipment. However, the invention is not restricted to this field of application; rather, it can be used in general whenever demagnetizing is to be performed using a demagnetizing coil.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.